A water jacket is formed around a cylinder bore of a cylinder block of a combustion engine, specifically a water-cooled engine; and cooling water, including cooling water mixed with antifreeze liquid, flows in the water jacket to cool a cylinder bore wall of which temperature increases while operating an engine. A water jacket spacer is inserted in the water jacket and appropriately cools the cylinder bore wall by adjusting the flow amount of cooling water, referring to the following Patent Literatures 1 and 2. Thus, appropriate cooling of the cylinder bore wall is done by the water jacket spacer. However, around a cooling water introduction port into the water jacket a lower portion of the cylinder bore wall is sometimes excessively cooled by the cooling water flowing into the back (on the cylinder bore side) of the water jacket spacer. When the lower portion of the cylinder bore wall opposite to the cooling water introduction port is excessively cooled, the viscosity of engine oil increases or the sliding resistance of a piston ring and a cylinder liner increases by deformation of the cylinder bore, thereby lowering energy efficiency. Patent Literatures 1 and 2 propose a structure for preventing excessive cooling of the cylinder bore wall around the cooling water introduction port (region opposite to the cooling water introduction port).
Patent literature 1 discloses, as a structure for preventing the above-mentioned excessive cooling, a seal structure for preventing water flow between the cylinder bore wall and the water jacket spacer, a structure in which the cylinder bore wall and the water jacket spacer are directly and firmly attached, a structure in which the water jacket spacer is energized or pressed against the cylinder bore wall and further discloses a structure in which the thermal conductivity of the cylinder bore wall around the cooling water introduction port is reduced. Patent Literature 2 discloses a structure in which an extending portion vertically along the cylinder bore wall is provided around the cooling water introduction port of the water jacket spacer and a portion bending from the extending portion is further provided, thereby inhibiting the flow of cooling water into the cylinder bore wall.